That's the big detriment to the evil path, none of them actually realise they've got a tadpole sitting on their brain that could turn them into a illithid at anytime. There's also pretty good indications that they're all being manipulated to varying degrees by the tadpoles and whatever is controlling them. Your character's tadpole is different because someone or something tampered with it but on that we have no idea why and there's no indications that the cult's master(s) are behind it. Even for an evil character, except for the psychotic, your best bet is to aid the grove and rescue Halsin. Now you don't have to help the Tieflings but getting to Halsin is presented as your best option with all others getting progressively worse.
